[clang] [Clang] Fix template elision to preserve qualifiers in diagnostics (PR #180175)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: lambo (Lambo-IITian)

<details>
<summary>Changes</summary>

### Description
Fixes #<!-- -->180046 

This PR addresses issue #<!-- -->180046 where Clang elides the wrong part of template types in error messages, specifically hiding important type qualifiers (like `const` or `volatile`) behind `[...]`.

The fix ensures that when template types differ only by qualifiers, those qualifiers are preserved in the diagnostic output rather than being elided, making the error messages much more actionable.

## Before changes error 

<img width="1740" height="267" alt="Screenshot 2026-02-06 172130" src="https://github.com/user-attachments/assets/ef3d2d99-d2b1-4cde-8017-b41142004069" />

## After changes error 

<img width="919" height="348" alt="Screenshot 2026-02-06 172337" src="https://github.com/user-attachments/assets/c458b47b-ef8a-4d07-84fa-677cce4be271" />


---
Full diff: https://github.com/llvm/llvm-project/pull/180175.diff


1 Files Affected:

- (modified) clang/lib/AST/ASTDiagnostic.cpp (+9-2) 


``````````diff
diff --git a/clang/lib/AST/ASTDiagnostic.cpp b/clang/lib/AST/ASTDiagnostic.cpp
index b8023cb6fa10f..7c498bc9b8e8a 100644
--- a/clang/lib/AST/ASTDiagnostic.cpp
+++ b/clang/lib/AST/ASTDiagnostic.cpp
@@ -1205,8 +1205,15 @@ class TemplateDiff {
              "Both template specializations need to be valid.");
       Qualifiers FromQual = FromType.getQualifiers(),
                  ToQual = ToType.getQualifiers();
-      FromQual -= QualType(FromArgTST, 0).getQualifiers();
-      ToQual -= QualType(ToArgTST, 0).getQualifiers();
+      // FromQual -= QualType(FromArgTST, 0).getQualifiers();
+      // ToQual -= QualType(ToArgTST, 0).getQualifiers();
+      bool Same = false;
+      if (FromArgTST->getTemplateName().getAsTemplateDecl() == 
+          ToArgTST->getTemplateName().getAsTemplateDecl()) {
+        // If the names match, the ONLY thing that makes them different is the Qualifiers
+        Same = (FromQual == ToQual); 
+      }
+      Tree.SetSame(Same);
       Tree.SetTemplateDiff(FromArgTST->getTemplateName().getAsTemplateDecl(),
                            ToArgTST->getTemplateName().getAsTemplateDecl(),
                            FromQual, ToQual, FromDefault, ToDefault);

``````````

</details>


https://github.com/llvm/llvm-project/pull/180175